1. Field of the Invention
The present invention relates to an apparatus for detecting positions. More particularly, the invention relates to an apparatus for detecting positions in a semiconductor aligner employed for the manufacturing process of semiconductor integrated devices, particularly of its lithography process.
2. Related Background Art
Traditionally, in an apparatus for detecting positions of an imaging detection type in a semiconductor aligner of the kind, the image of an interleaving mark (index pattern) on a reticle and the image of a wafer alignment mark are imaged on a image sensor through a projection optical system and imaging optical system, and a positional detection is performed on the basis of image signals which vary its level in accordance with the marking images from the image sensor (through-the-reticle method) . Also, the index pattern on the index plate which is provided in a substantially conjugate relationship with a wafer in an alignment optical system through an objective optical system which is different from the projection optical system and the image of a wafer alignment mark are imaged on an image sensor through an imaging optical system, and a positional detection is performed on the basis of image signals which vary its level in accordance with each image of the marks and patterns from the image sensor (off axis method).
In reference to FIGS. 6A through 6E, the description will be made of a conventional apparatus for detecting positions.
At first, the through-the-reticle method will be described. FIG. 6B illustrates a conventional index pattern AM1. FIG. 6C illustrates a conventional alignment mark WM. FIG. 6D is a view showing signal waveform data from an image sensor. The axis of ordinate represents signal levels while the axis of abscissa represents mark (or pattern) positions. The index pattern AM1 is formed by edges Ega on both ends of the opaque portion as its index patterns. The opaque portion Cr is formed by a transparent material such as quartz coated with chrome or the like. The alignment mark WM in FIG. 6C is assumed to be a three-piece multimark formed by line and space having its duty ratio of 1:1 in the direction of measurement.
Now, the waveform data from the image sensor become as shown at SIG1 in FIG. 6D when the index pattern AM1 and the alignment mark WM are overlapped at a designed position. Among the waveform data at SIG1, the six wave troughs WM1 to WM6 of the WM data corresponding to the alignment mark WM correspond to the edge positions, and the wafer position is determined on the sensor. On the other hand, the two wave troughs of the AMD1 data corresponding to the edge portions at the two locations correspond to the respective edges Ega of the index pattern AM1, and the position of the index pattern is determined on the sensor from these data. More specifically, two positional data are obtained using a slope 1 from the opaque portion to the trough AMa or a slope 2 from the trough AMa to the top of the wafer signal (WM data), and using these data, the pattern position of the index pattern AM1 is determined.
Also, in the off axis alignment method, a linear opaque index pattern as shown in FIG. 6A is provided on a portion of a transparent glass plate, which corresponds to the edges Ega in FIG. 6B. Then, this index plate made of glass plate is provided through an objective lens at a position substantially conjugate with the wafer, and the transparent portion of the index plate and opaque index pattern are illuminated. Therefore, as shown in FIG. 6E, it is possible to obtain such signal as at SIG1' which is similar to the one at SIG1 when the alignment mark WM is positioned between the index patterns AM1. Hence each of the positions being determined.
However, according to the conventional technique described above, the image of the interleaving mark (index pattern) on the reticle or the image signal waveform corresponding to the image of the index pattern in the alignment system in an off axis method is fluctuated depending on the reflection factor of the wafer, the irregularity of the wafer surface, or the like. For example, the intensity difference h.sub.2 of the slope 1 and the intensity difference h.sub.1 of the slope 2 of the waveform data SIG1 and SIG1' shown in FIG. 6D and 6E, or the inclinations of the slopes 1 and 2 are sensitively fluctuated in response to the difference in the reflection factor of the wafer surface, irregularity of there, or the like (the wafer surface conditions), or the slopes 1 and 2 are distorted depending on the irregularity of the wafer surface.
As a result, it is impossible to detect the pattern positions accurately with a conventional index pattern such as this. Also, there is a problem that a positional detection cannot be performed with a desirable reproducibility.